The present invention relates to a dual-voltage vehicle electric system according to the preamble of patent claim 1.
The voltage supply in motor vehicles has conventionally usually been provided using a single battery which is charged by a generator. In modern motor vehicles having a multiplicity of electric loads one battery is in some cases no longer sufficient to supply voltage, for example to ensure starting, so that two separate batteries are used. In this context, one battery is generally assigned to the starter and the other to the rest of the vehicle electric system. For an emergency operating mode it is also possible to switch to the respective other battery in order to permit starting even when a battery is severely discharged.
For example, DE 198 13 369 A1 discloses a dual-battery vehicle electric system which has a circuit arrangement which is located between the two batteries, specifically a vehicle electric system battery or load battery and a starter battery, and can connect them to one another. If the load battery is discharged excessively, a relay opens a connection between the load battery and loads which are relevant to starting, and a semiconductor switch is switched in such a way that the entire voltage supply is drawn from the starter battery as an emergency operating mode for the loads which are relevant to starting.
Furthermore, DE 196 45 944 A1 discloses a vehicle electric system having at least two batteries which can be charged by a generator and which serve to supply various loads. A vehicle electric system control unit opens and closes a connection between the two batteries, one battery being provided predominantly for supplying the starter while the second serves to supply the other loads. If the voltage level of one of the two batteries drops below a predefinable value, an emergency operating mode can be implemented in which the starter battery can be charged from the other battery via a DC voltage converter to such an extent that restarting is ensured.
DE 196 28 222 A1 discloses the dual-battery vehicle electric system in a motor vehicle in which a vehicle electric system control unit is arranged between the batteries and one battery is predominantly used to supply power to the starter. The vehicle electric system control unit comprises a vehicle electric system module which carries out the necessary switching processes and has another terminal via which vehicle electric system components which are necessary for starting and for operation can be supplied with voltage. These vehicle electric system components are supplied with power with the highest priority, and are also supplied from the starting battery if the supply battery is excessively discharged, but in this case it is ensured that the other vehicle electric system components are not connected to the starting battery during the starting process.
DE 90 07 413 U1 also discloses a vehicle electric system which has a power supply component 11 with two batteries B1, B2 which are connected in series and have the purpose of supplying power to loads v1 to Vn, and a switch device 21 having at least one relay-actuated switch 31 and one electronic switching control device 61. By comparison with a defined voltage limiting value when the first battery B1 drops below this voltage limiting value, the switching control device 61 controls, on the basis of the voltage potential fed to it from one battery or both batteries B1, B2, the switching of the loads V1 to Vn from the potential of the first battery B1 to the overall rest voltage potential of both batteries B1, B2 by correspondingly energizing one of the relays 71 which activates the switches 61. The design of this conventional vehicle electric system is shown in FIG. 2.
However, in such systems, switching over to the starter battery as an auxiliary battery occurs only when the voltage of the vehicle electric system battery drops below a voltage limiting value. In addition, just one operating voltage is available for the vehicle electric system. However, high performance loads which are to be used in the vehicle electric system in future will require a higher voltage level than the loads which are conventionally present in the vehicle electric system.
DE 38 12 577 A1 discloses, for example, a vehicle electric system for a motor vehicle in which the voltage supply is provided using two generators and two batteries which are connected in series. The rated voltage of the first generator or of the first battery is different from the rated voltage of the second generator or second battery, so that in total three different voltages can be tapped at the series circuit comprising the two batteries. By using the two generators it is possible to charge the two batteries to an optimum degree even when there is uneven loading on the vehicle electric system. In addition, by way of example, voltage-sensitive components, for example for the ignition and/or injection, are connected to one of the batteries, and the starter is connected to the other battery with the higher voltage, as a result of which a voltage dip during starting does not disrupt the voltage-sensitive components.
Furthermore, DE 102 08 981 A1 discloses a vehicle with two vehicle electric systems and two batteries with different voltage levels. In this context, each vehicle electric system has a battery assigned to it. Furthermore, two intelligent power distributors with a power management facility for the various loads are provided and they monitor the entire vehicle electric system, including the battery, and for the respective vehicle electric system they process the input current and output current and the state of charge of the two electric systems by means of the power management facility.
Finally, DE 101 00 888 A1 discloses a vehicle electric system which has at least two batteries which can be charged by a generator and which are used to supply power to a first load and a second load. One of a plurality of DC voltages can be optionally generated by means of a DC voltage converter. The respectively generated DC voltage is made available for charging the second battery.
However, these two dual-voltage vehicle electric systems always require at least two batteries with different voltages. For this reason, at least one battery with a voltage higher than in conventional vehicle batteries is necessary, and such a battery requires more installation space and entails higher costs.
The object of the present invention is to provide a dual-voltage vehicle electric system which can be constructed with cost-effective conventional components and which requires as little installation space as possible and at the same time has the increased availability of a dual-battery vehicle electric system and is capable of emergency starting and emergency operation.